Synthetic polymers are widely used as sizing agents for cellulose-based products, e.g. paper, paperboard, etc. When added or applied during or at the conclusion of the papermaking process, sizing agents generally improve paper properties such as printability by imparting a more hydrophobic character to the surface of the paper and thereby preventing or reducing the flow of ink into or across the surface of the paper. The polymers that impart these characteristics to the paper generally have a hydrophobic nature and therefore may be soluble or insoluble in aqueous solution, depending on the level of hydrophobicity incorporated. When soluble in water, they may be supplied to the user in the form of an aqueous solution. When insoluble in water, they may be conveniently supplied to the user in the form of aqueous dispersions or emulsions in which small particles or droplets of the sizing polymer are dispersed throughout an aqueous solution.
These aqueous dispersions or emulsions are generally formed by polymerizing or copolymerizing monomers in the presence of water. To facilitate the formation of a dispersion or emulsion and to prevent the forming polymer from immediately coagulating into a large mass, emulsifiers and surfactants are frequently present during the polymerization to aid in the formation of the dispersion and/or stabilize the final product. For instance, in WO 97/37078 it is stated that anionic, cationic, amphoteric and nonionic emulsifiers can be used. However, in some cases the use of surfactants and/or emulsifiers can lead to undesirable foaming and lower sizing when present during the papermaking process.
Many conventional polymeric sizing agents contain recurring units of an ethylenically unsaturated comonomer which contains carboxylic acid or sulfonic acid groups or salts thereof, see e.g. WO 97/37078 and U.S. Pat. Nos. 5,240,771; 5,231,145; 5,139,614; 5,138,004; 4,115,331; 4,030,970; and 4,001,193, all of which are hereby incorporated herein by reference. However, since the salt and acid forms often have different solubilities, the inclusion of these recurring units in surface sizes may have the undesirable effect of increasing the pH sensitivity of the sizing agent, in some cases so much so that the ability of the sizing agent to function for its intended purpose is greatly impaired.
In recent years, other sizing agents e.g. styrene/acrylate emulsions have been developed which do not include recurring units of an ethylenically unsaturated comonomer which contains carboxylic acid or sulfonic acid groups or salts thereof. However, even for sizing agents in which inclusion of these units is optional, see e.g. U.S. Pat. No. 4,835,212, there remains a problem in that further improvements in sizing ability and/or dispersion stability are desired.
Thus, there is a problem in that existing sizing agents often contain undesirable levels of surfactants, emulsifiers and/or acidic recurring units, see e.g. U.S. Pat. No. 5,258,466. The simple expedient of removing these components from the formulation has proven to be unsatisfactory because elimination often causes undesirable reductions in sizing ability and/or dispersion stability. Therefore, the problem of reducing surfactant, emulsifier and/or acid component levels in sizing agents is complicated by the need to retain or improve sizing ability and dispersion stability.
Surprisingly, it has now been discovered that sizing agents may be prepared by polymerizing the monomer components of a mixture of monomers, where the mixture contains effective amounts of methyl acrylate, ethyl acrylate or propyl acrylate, and a stabilizing agent in the presence of water and in the absence of both (a) ethylenically unsaturated carboxylic and sulfonic acids and salts thereof and (b) emulsifier or surfactant having a molecular weight of less than about 1,000, to give dispersions having the ability to impart a degree of sizing to cellulose-based products that is as good as or better than comparable products which contain acidic recurring units and/or emulsifier or surfactant and/or lack effective amounts of methyl acrylate, ethyl acrylate or propyl acrylate. Surprisingly, the sizing dispersions of the instant invention also exhibit the improved physical stability often associated with ethylenically unsaturated carboxylic and sulfonic acids and salts and/or emulsifier or surfactant without the use of these components.